A free virtual protractor that measures angles in degrees on your screen or over any uploaded image — no download, no sign-up.Drag the arms to measure angles on photos, blueprints, geometry worksheets, or technical drawings. Supports 180° and 360° modes.
This tool renders an interactive, draggable protractor on your screen using HTML5 Canvas. You position the central origin point at the vertex of any angle, then drag two arm handles along the lines forming the angle. The tool computes the angle using vector mathematics — specifically the atan2 functionapplied to the difference in screen coordinates between the origin and each arm handle — and displays the result in real time. It supports both 180° (semicircle) and 360° (full circle) modes, and can be overlaid on any uploaded image.
| Scenario | Online Protractor (ToolsPixel) | Physical Protractor |
|---|---|---|
| Measuring angle on a photo/diagram | ✅ Best choice | ❌ Not possible |
| Geometry homework | ✅ Instant, free, no tools needed | ✅ Works fine |
| Woodworking cut verification | ✅ Upload photo and measure | ✅ Direct measurement |
| Precision engineering (±0.1°) | ⚠️ ~±0.5–1° accuracy | ✅ Calibrated instrument needed |
| No physical tool available | ✅ Works on any device | ❌ Must own one |
| Reflex angles >180° | ✅ 360° mode available | ⚠️ Requires manual calculation |
Students and teachers. Geometry students use this to check angles on printed or PDF worksheets, verify triangle interior angles, and explore supplementary/complementary pairs interactively.
Woodworkers and carpenters. After photographing a joint or template, craftspeople upload the image and measure angles directly over it to plan cuts.
Photographers and videographers. Photographers measure composition angles on reference shots to replicate them consistently.
Interior designers and architects. Checking the angle of a feature wall, staircase rake, or roof pitch from a photograph is straightforward — upload the image and measure the relevant angle.
Engineering students. Technical drawing and statics problems frequently require measuring angles on force diagrams. Students upload a PDF screenshot and measure directly on screen.
The protractor computes angles using standard 2D vector mathematics. Each arm defines a vector from the origin point to its handle position. The angle between two vectors is calculated using: angle = Math.atan2(vy, vx) for each arm, and the angle between them is the difference of these two arctangent values, converted from radians to degrees.
The Math.atan2(y, x) function is used instead of Math.atan(y/x) because it correctly handles all four quadrants of the coordinate plane and returns values in the full −π to +π range, making it reliable regardless of which direction the arms point. The result is normalized to the 0–360° range for display.
Human alignment is the limiting factor: The math is exact, but how precisely you position the arm handles over the lines in an image determines practical accuracy. Typical accuracy is ±0.5°–1° with careful positioning.
Screen resolution matters: On higher-resolution screens, you can position handles more precisely. On low-DPI displays, coarser pixel steps reduce alignment precision.
Not a substitute for a precision instrument: For engineering or construction where angle tolerances are tight (±0.1° or less), use a calibrated digital angle gauge. This tool is excellent for visual checking, education, and planning — not for precision manufacturing.
Learn how to take accurate measurements directly on your phone or computer screen, using calibration techniques that work with the protractor too.
How to Measure Objects with Your Phone Screen (Accurately) →Online Measuring Tape
Measure small physical objects on your screen with a calibrated ruler.
Dead Pixel Checker
Test your monitor or smartphone display for stuck or dead pixels.
Screen Recorder
Record your desktop screen, windows, or browser tabs online.
Online Compass
Find your direction online using your mobile device's compass sensor.